Treasury secretary Martin Parkinson has launched a staunch defence of his department’s economic forecasting and “completely and utterly” rejected charges that last week’s budget was compromised by shuffling spending and savings to conjure a surplus.

In a wide-ranging speech in Sydney, Mr Parkinson said greater uncertainty about revenue and structural changes within the economy made generating accurate forecasts more difficult.

He attacked claims that the forecast swing from deficit to surplus means fiscal and monetary policy are in conflict, saying the Reserve Bank of Australia had scope to lower interest rates if the economy weakened.

“The idea that somehow there is a tablet that says how one constructs your budget – and that somehow . . . the Treasury have done something inappropriate this year – let me just reject that completely and utterly,” Mr Parkinson told a gathering of economists.

The practice of shifting items between accounting years happened in every budget and reflected changed circumstances such as the early completion of an upgrade of the Ipswich motorway that allowed the Treasury to move spending originally slated for 2012-13 into 2011-12, he said.

There has been criticism of Treasury for overestimating economic growth in the current financial year and for making assumptions about a likely rebound in government revenue for next financial year. The forecasts have been described by at least one market economist as the most optimistic in half a century.

Sydney University economics professor and former Reserve Bank board member Adrian Pagan last week accused the Treasury of allowing its budget forecasts to be manipulated in 2009, likening them to Greece’s public accounts, a charge the department vehemently rejected.

“Given the significant structural change in the economy, and the changing relationship between the nominal economy and tax collections, this is a particularly challenging time for revenue forecasting,” Mr Parkinson said in his speech.

Uncertainty caused by a series of shocks to the economy, including the global financial crisis and the resources investment boom, had made the task more difficult and that small errors could have a material effect on the accuracy of overall forecasts, he said.

In last year’s budget, the government pencilled in import growth of 10.5 per cent for 2011-12, less than the 12.5 per cent now thought likely. It anticipated export growth of 6.5 per cent. Treasury now expects a 4 per cent gain.

Those mis-steps meant that net exports detracted more from gross domestic product growth than had been expected in the 2011 budget. Driving this error was an underestimation of the effect of surging business investment growth on imports.

“As you know, resources investment is very import intensive, and the shift towards investment for LNG projects makes it even more so,’’ Mr Parkinson said.

“Again, this is not much of a miss, given the size of the growth rates. But with imports also equivalent to around 20 per cent of GDP, it still has a material effect on the accuracy of the forecasts.’’

Mr Parkinson signalled further misses given that the mining boom had caused things to move “around much more than usual’’.

Treasury expects business investment to grow by well over a fifth this year, alongside a 12 per cent gain in imports and a 10 per cent drop in the terms of trade.

“Such large movements are difficult to calibrate tightly, but small errors in those components can make a big difference to the accuracy of the forecasts,’’ the Treasury secretary said.

Mr Parkinson used his speech to launch a fresh defence of Australia’s macro-economic framework, saying he found it “disheartening” to hear criticism from parts of the business community about a system that had served Australia well.

“Unfortunately, it is sometimes forgotten that this framework is in no small part responsible for the relative stability of Australian economic growth,’’ even through three of the biggest shocks since World War II to have hit the nation in the past decade.

Those shocks included the surge in demand for mineral resources between 2004 and 2009, the global financial crisis, and the renewed resources boom since then.

Now that the economy is forecast to return to its average growth rate, and unemployment is expected to be near its lowest sustainable rate, it was appropriate for the budget to return to surplus, he said.

“So, rather than working at cross-purposes, the current directions of monetary and fiscal policy simply reflect a return to their normal roles following an extreme event.”

In last week’s budget, Treasury forecast a return to average growth rates, driven by the resources boom, as well as higher domestic demand triggered by cuts in interest rates.

Mr Parkinson said that while the swing from deficit to surplus implied a fiscal consolidation of 3.1 per cent of GDP, the macro­economic impact was “probably under’’ 1 per cent of GDP.